1. Field of the Invention
This invention relates to adipic acid-modified-ionomers. The modified-ionomers have improved processability as well as certain improved properties and to golf balls and golf ball covers containing these modified ionomers.
2. Background of the Invention
Copolymers which are dipolymers of ethylene and the unsaturated carboxylic acids methacrylic or acrylic acid, are well known. Commercially such dipolymers typically contain at least about 75 weight percent, and up to about 96 weight percent, ethylene. They are used for molding, packaging and some adhesive applications. Examples of such ethylene/acid-copolymers include those sold under the trade name, Nucrel.RTM., which are produced by E. I. du Pont de Nemours and Company. Such ethylene/acid-copolymers are disclosed in U.S. Pat. No. 4,351,931 (Armitage), the parent disclosure of which was filed in 1961.
Acid-copolymers of this type are also used to prepare `ionomers`, wherein the carboxylic acid units of the copolymers are partially neutralized with metal ions. Such ionomers are sold by E. I. du Pont de Nemours and Company under the trade name Surlyn.RTM.. These resins are thermoplastic in the melt. Partial neutralization is used because fully neutralized acid-copolymers are known to, and are disclosed as having, intractable melts. Ionomers have separate utility and significantly different properties from the non-neutralized ethylene/acid-copolymer `precursors`. Ionomers were first disclosed in U.S. Pat. No. 3,264,272 (Rees). The ions form a sort of `ionic crosslink` bond at low temperatures, but the ions in these ionic crosslink bonds are sufficiently labile at melt temperatures, provided there remain some non-neutralized acid groups, that they allow thermoplastic melt processability. Nevertheless, ionomers are considerably more viscous than their acid-copolymer precursors.
Typically, ionomers are made from acid-copolymer precursor copolymers having a melt index of from about 20 to 300 grams/10 minutes. Neutralization results in an increase in viscosity, or a decrease in melt flow, commonly measured as melt index (MI). Useful ionomers can have a level of neutralization of between 10 and 90 percent, preferably 25 and 75 percent, and their MI falls between 0.1 and about 20 grams/10 minutes, but preferably lower than about 3 grams/10 minutes. The higher the acid-copolymer precursor MI, the higher the MI of the ionomer for a given level of neutralization percent of acid groups neutralized). Higher MI generally allows more ready processing. Nevertheless, many of the attractive properties associated with ionomers compared with acid-copolymers depend on higher levels of neutralization, and hence poorer flow. In addition, lower MI of the acid-copolymer precursor is often more desirable because this relates to the underlying polymeric chain length, which ionic crosslinking does not change significantly. The reality is that neutralization to an MI of 3.0 or less is highly desirable and ionomers with MIs of less than 1.0 have particularly good properties. At these MI levels however, processability is considerably reduced compared with higher MI ionomers.
U.S. Pat. No. 4,104,216 (Clampitt) describes ionomers modified with 5-50 weight percent of a 19-11C long chain (un)saturated fatty acid as `plasticizer`. The product has higher MI, but lower glass transition temperature, and lower stiffness.
Japanese patent application No. 48/70757 discloses ionomers modified with a high level of a low molecular weight saturated or unsaturated carboxylic acid or salt or anhydride, specifically 10 to 500 parts per 100 parts by weight of ionomer. The carboxylic acid may have 1 to 100 hydrocarbon carbon chain units. Stearic, citric, oleic and glutamic acid and/or salts are exemplified. The acid compounds specifically disclosed include 38 carboxylic acids or metal salts, but do not include adipic acid. The useful additives are likened to plasticizers, comparison being made with plasticization of poly(vinyl chloride) having `appropriate flexibility` and improved processability at high temperatures.
There is a need for ionomers which have improved processability and other properties, but which are not plasticized in the sense of reduced modulus and stiffness, and which do not have deficiencies in other properties.
In addition, Synthetic balata is used as a soft cover material for golf balls to provide a soft feel, high spin and good ball control characteristics. It is high cost, requires crosslinking and is deficient in cut resistance, tear strength, yield behavior and scuff resistance. Current ionomer technology using such brands as SURLYN.RTM. by E.I. DuPont de Nemours and Company allows or provides balls with covers having good toughness properties, cut resistance and yield behavior but does not provide the soft feel of balata. There is a need therefore to develop golf ball resins or formulations having all the beneficial properties of ionomer resins currently sold as used for this purpose but which also have the soft "feel" of the balata covered balls. The present invention meets this need by providing a formulation comprising an ethylene acid ionomer which provides good toughness properties, cut resistance and yield behavior and an adipic acid which diminishes the resilience of the composition to provide golf ball covers which mimic the soft "feel" of balata covered golf balls wherein the golf balls do not have any of the disadvantages of the balata covered ball.